Polyorganosiloxane base coating compositions are superior in heat resistance to coating compositions on organic polymers such as polyester and polyimide, but cannot withstand elevated temperatures of higher than 400.degree. C. for a long time.
In the recent years, there is an increasing demand for coating compositions capable of preventing oxidation and corrosion of metallic and non-metallic substrates which are serviced at high temperatures in excess of 1000.degree. C. It is also desired to develop coating compositions which form coatings maintaining electric insulation at high temperatures and having good adhesion.
A variety of heat resistant coating compositions have been proposed in the art. (1) Japanese Patent Application Kokai (JP-A) No. 54768/1987 discloses a composition comprising polytitanocarbosilane, silicone resin, and inorganic filler. (2) JP-A 235370/1987 discloses a composition comprising polycarbosilane, silicone resin, and inorganic filler. (3) JP-A 92969/1990 discloses a heat resistant coating composition having organometallic polymer and silicon dioxide blended therein. (4) Japanese Patent Publication No. 50658/1983 discloses a composition comprising borosiloxane resin.
These proposals, however, have some drawbacks. Heat resistant coating compositions (1) and (2) are unsatisfactory in adhesion to substrates at high temperatures, crack resistance of coatings, and high-temperature electric insulation. Heat resistant coating composition (3) suffers from separation and cracking of coatings at high temperatures and poor electric insulation. Heat resistant coating composition (4) is poor in water resistance and high-temperature electric insulation. The previously proposed approaches do not satisfy all the requirements of high-temperature adhesion, heat resistance, water resistance, and electric insulation. There is a need for developing a heat resistant coating composition capable of satisfying all such requirements.